1. Field of the invention
This invention relates to a servo tracking circuit used in an optically writable/readable/erasable disk apparatus.
2. Description of the Related Arts
An optically or magneto-optically writable/readable/erasable disk (referred to hereinafter as optical disk) apparatus has been popularly employed as a medium to record large amounts of digital data. A light emitted from a light source, typically a laser diode, must be focused onto a track of as fine as several micro meter spacing. In order for the light to trace the track, a tracking servo and a focusing servo are employed and a tracking error signal and focusing error signal obtained from a light reflected from the track is negatively fed back so as to become minimum.
On the other hand, level and duration of the light is modulated so as to be adequate to write, read or erase respectively the data on the disk. The light level to write or to erase the data is considerably larger than the level to read the data, therefore, by largely changing the light emission level, the oscillation mode of the laser diode changes. In addition, the modulation current applied to the laser diode changes abruptly and drastically. Furthermore, undesirable noise pulses are formed in the tracking/focusing error signals. These noise pulses further cause an erroneous operation of the tacking/focusing servos.
In order to avoid this erroneous operation there have been proposed methods where the tracking/focusing error signal is masked, i.e. disabled, during a period when the tracking/focusing error signal is higher than a predetermined threshold level (such as disclosed in Japanese Laid-open Provisional Patent Publication Hei1-263947) or during a predetermined period at transitions of the light levels (such as disclosed in U.S. Pat. No. 4,807,206 and Japanese Laid-open Provisional Patent Publication Hei2-66743). The predetermined period is determined by hardware such as a shift resistor or a monostable multivibrator.
The prior art methods experience problems in that the required period varies depending on the amplitude of the light modulation or the circuit condition, however, the period to be predetermined cannot be easily modified because the period is determined by the hardware. Furthermore, in tuning up the optical disk apparatus it is desirable that the period is easily adjustable.